I had about 40 kg of lead which I melted into a diamondplate box I made and bolted that into the frame I made for the transport hack, so it was under the flat floor.
I now have a "lastenseitenwagen" as hack and just bolted it to the floor, since it's pretty flat it doesn't take too much space.
This is next to my MZ 301 2-stroke.
On my R 90/6 with SuperElastik sidecar I have about the same weight in the boot, as far to the outside as possible, mounted with a bolt with a wingnut so I can take it out pretty easy. Just have to make some kind of grip on it so I can pick it easier
1300cc 217Nm/207Hp NOS-injected VMax (1985), 500 cc 1950 BSA chopper (1983) in pieces, 750 cc CB750 chopper (1990), 300 cc MZ transporthack (1989) winterhack, 900 cc BMW R 90/6 hack (1976) summerhack, 1100 cc BMW R1100GS (1997) daily driver.
Age is an issue of mind over matter. If you don't mind, it doesn't matter ( Mark Twain )